Tunable external-cavity semiconductor diode lasers (ECDL) including frequency selective tuning elements in the cavity are gaining wide spread use in many applications and notably in communications. In addition to lenses, mirrors, filters and various other components, such lasers typically include a semiconductor high gain media having an antireflection (AR) coating on the intracavity facet to reduce its reflectivity. Other components of the laser may also include AR coatings.
Whenever light crosses the boundary between two media some or all of the light may be reflected. The refractive index, traditionally designated “n”, is a measure of the amount of impedance a particular material has to the propagation of light. In a vacuum light travels at a rate of 3×108 m/s. When light travels through a different medium, the “impedance” of that medium slows the speed of the light to c/n, where “c” is the speed of light in a vacuum and “n” is the refractive index.
The AR coating operates to match the impedance (or admittance,the reciprocal of impedance) between the gain media and the surrounding media. The surrounding media is typically air having a refractive index of n about equal to 1, very close to that of a vacuum. The amount of light reflected is dependent on several factors including the wavelength of the light, polarization, the reflective indices of the media, and the incident angle of the light itself.
In many applications it is desirable that as much of the incident light as possible be transmitted from the gain media and not reflected away. Unfortunately, the reflection at a boundary between air and a typical gain medium material is about 30% of the incident light. Perhaps more significant for ECDLs, it is desirable that as little light as possible be reflected back into the gain media. This back reflection is generally referred to as feedback and has a destructive interference effect on the laser output. Generally the feedback should be kept less that −40 dB (i.e., less than 0.01%). Depositing an efficient AR coating on the gain media surface can significantly reduce reflection.
As noted above, one of the factors affecting the amount of reflection is the wavelength of the light. So to, the wavelength of the light affects the performance of the AR coating in its ability to mitigate reflection. Tunable lasers are expected to operate over a wide range of frequencies or wavelengths. Thus, the AR coating used should be selected to operate effectively over a wide range. A judicious choice of the material used for the AR coating may lead to significant broadening of the AR performance. However, this often requires the use of somewhat exotic materials to realize the best effect. In general, these materials may not even exist. In practice, an AR coating comprises at least two or more dissimilar materials layered, one on top of the other, in order to improve the overall AR effect. However, this solution typically improves performance over only a narrow range of frequencies and broadband performance of the laser suffers outside this range.